The field of the invention relates to a control system for maximizing power output of internal combustion engines. In particular, the invention relates to controlling the timing of combustion for maximizing engine torque output. Combustion timing may be controlled by controlling ignition timing in one embodiment and fuel timing in another embodiment.
It is known that optimal torque output, within acceptable emission limits, is achieved when ignition timing of an engine is set at "minimum spark for best torque (MBT)." The ignition timing of a particular model of motor vehicle is typically set or calibrated at a predefined spark advance before top dead center (TDC) such that the average of all such vehicles, when new, is near MBT. This approach however has been found to be less than optimal because variations among engines, subsequent maintenance, environmental conditions, and aging often result in an actual MBT which is different from the initial spark advance calibration. It is therefore desirable to have a control system which continuously maintains ignition timing at MBT.
Various approaches have been attempted to achieve MBT during vehicular operation. In one typical approach, pressure transducers are coupled to the combustion cylinders and the crank angle location of peak pressure (LPP) is compared to a reference. Allegedly, the reference is associated with MBT. It is further presumed that LPP always provides a measurement of MBT. Examples of these approaches are found in U.S. Pat. No. 4,063,538 issued to Powell et al, U.S. Pat. No. 4,391,248 issued to Latsch, U.S. Pat. No. 4,481,925 issued to Karau et al, U.S. Pat. No. 4,706,628 issued to Trombley and U.S. Pat. No. 4,760,825 issued to Morita.
A disadvantage of the above approaches is that actual LPP varies with changing operation conditions. That is, the correlation between MBT and LPP is not constant. It is therefore contented that a fixed reference value for LPP does not exist. Another disadvantage is that LPP does not always provide a measurement of MBT in such engines as engines with stratified fuel charges. Accordingly, these types of control systems have been found to be unsatisfactory.
Another approach aimed at achieving MBT is to change spark ignition timing and compare fluctuations in engine speed. Examples of this approach are found in U.S. Pat. No. 4,026,251 issued to Schweitzer et al, and U.S. Pat. No. 4,379,333 issued to Ninomiya et al. In U.S. Pat. No. 4,026,251 ignition timing is perturbed to one side of the reference for a predetermined sample time and engine speed measured. Ignition timing is then perturbed to another side of a reference value for a predetermined sample time and engine speed again measured. Ignition timing is then adjusted in response to the measured difference in engine speed. The inventor herein has recognized numerous disadvantages of this approach. For example, during the sample times, engine speed may vary as a result of changes in operational conditions such as while encountering inclines. Further, engine speed at a particular sample time is also related to engine time at a previous sample time due to inertial effects. Thus, changes, in engine speed are caused by more factors than the perturbation and ignition timing. Accordingly, an accurate measurement of changes in engine torque resulting from changes in ignition timing may not be obtainable. In addition, a predetermined number of samples must be taken at each ignition timing offset as determined by worst case analysis. Stated another way, each ignition timing decision, or correction, occurs at a fixed time interval which is determined by the worst case conditions under which such a decision may be made. These systems have therefore also been found to be unsatisfactory.